How to align 2D UV map for wood slat ceiling
Maybe a screenshot of your UV Editor would help to understand where the problem comes from.
Here is a try, I've unwraped the mesh, aligned the 2 parts and inverted one with SX-1. The small problem is that the planks may look a bit the same:

A solution would be to shift the UV a bit, it will work fine if all your planks have about the same width, otherwise they won't align anymore: